1. Field of the Invention
The present invention relates to a system for obtaining and processing well log data and, more particularly, to such a system for determining on a real time basis seismic wave velocities in a computationally efficient manner.
2. Setting of the Invention
Geophysicists, geologists, and the like are interested in obtaining seismic wave velocities, since with these velocities one is better able to determine the likelihood of hydrocarbons being present within a given formation. Seismic wave velocities can also be used in designing fracture treatments for hydraulically fracturing a borehole to increase the permeability of the particular formation. Sonic logging can be a source of these seismic wave velocities because sonic logging provides a measurement of certain properties of a formation material around a borehole by the measuring of the velocities (or slownesses) of various seismic waves which travel through the earth, as well as certain waves guided by the borehole interface. Basically, sonic logging involves imparting seismic or acoustic energy into a borehole wall at one point and the reception of part of that energy, which has been transmitted back into the borehole, at another point. With a known distance between the transmitter and one or more receivers, the time lag between transmission and reception can be used to indicate the seismic energy's velocity or slowness (1/velocity).
In conducting sonic logging operations, it is desirable to obtain the processed results, for example, the seismic wave velocities as soon as possible so that any additional logging can be accomplished while the logging equipment is still at the well site; or, more significantly, so that hydrocarbon-bearing zones can be located and completed. However, currently the usual turnaround time, i.e., the time from logging to receipt of the processed data, is on the order of about one week to about one month. This delay is caused in part because once the logging data is obtained at the well site, it is transported or transmitted to a remote processing location. Due to the complexity of the algorithms needed to process the data, the data is processed only on large mainframe computers. The lack of real time processing, defined here as the ability to obtain processed data for interpretation from the sonic well data either as the data is being generated or immediately thereafter, generally limits the quality control of the log during the data acquisition. This quality control is a vital aspect of any logging procedure, since with real time processing one can relog a well, if needed, before the well is altered by production treatments and a log can be rerun while the required logging equipment is still at the well site.
Various sonic logging processes have been disclosed in the past; however, none of the processes known to the inventors hereof have the capability of real time processing of the data. Also, no known process uses a computationally efficient algorithm so that the data can be processed by a very easily transportable and inexpensive hardware system, such as by a microprocessor, rather than being post-processed on a large mainframe computer at a remote computing facility.
Two patents which disclose correlation techniques to derive various seismic wave velocities from sonic logging data are Ingram, U.S. Pat. No. 4,210,966, and Seaman, U.S. Pat. No. 4,367,541. Ingram discloses a correlation technique for determining acoustic wave velocities from sonic logging data and requires a point-by-point multiplication of successive different wave form segments for different assumed wave velocities to derive an acoustic wave velocity that produces the best correlation between successive waveform segments. This is a correlation technique which is not easily handled by small computers, such as a microprocessor because it is not a computationally efficient program. Further, there is no disclosure or suggestion within Ingram of obtaining the seismic velocity data on a real time basis, using a nonlinear Nth root stacking algorithm, or using an increasing sloped window line in stacking the signals.
Seaman discloses a method and apparatus for selecting an acoustic wave velocity from a plurality of provisional wave velocities. More particularly, acoustic wave energy from a transmitter positioned within the borehole is received at a plurality of spaced locations within the borehole. The waveform segments received at each receiver are then correlated using the Ingram technique to derive a first provisional velocity. However, unlike Ingram, a second iteration of the correlation technique of Ingram is performed on a second segment of the waveform to derive a second provisional velocity. Seaman then provides a means for selecting a final output velocity as a function of the two provisional velocities derived utilizing the Ingram correlation technique. However, Seaman indicates that various correlating techniques are available in the art and could be utilized, hence implying that the essence of Seaman is not directed toward any particular technique for correlating, but rather only a process for selecting a final output velocity from the provisional velocities derived from iterations of the Ingram technique.
Nowhere is it disclosed or suggested within Ingram or Seaman to correlate the signals from the various receivers utilizing a computationally efficient algorithm that determines the seismic wave velocities by measuring the coherency of the signal in a nonlinear fashion. This nonlinear process, as well as the minimum multiplication steps within the algorithm provide a computationally efficient algorithm that allows real time data processing, which has henceforth been unavailable. Further, there is no disclosure or suggestion of using a nonlinear Nth root stacking algorithm, or using an increasing sloped window line in stacking the signals.
Other references of note are U.S. Pat. Nos. 3,696,331; 3,424,268; 3,390,377; 3,292,729; and 3,177,467. All of these references disclose various techniques for deriving acoustic wave velocities; however, none of these references disclose or suggest a computationally efficient method for determining the wave velocities without multiplicative iterations. Also, various techniques of crosscorrelating signals to ascertain wave velocities are discussed in U.S. Pat. Nos. 3,962,674; 3,900,824; 3,696,331; and 3,622,969. Additionally, UK Pat. No. 2,107,462A; 2,111,206A; and U.S. Pat. No. 4,414,651 disclose other techniques of cross correlation to determine acoustic wave velocities. However, nowhere in any of these references is it disclosed or suggested to use a computationally efficient processing technique to obtain seismic wave velocities on a real time basis as the data is gathered. Further, there is no disclosure or suggestion of using a nonlinear Nth root stacking algorithm, or using an increasing sloped window line in stacking the signals.